The present invention relates to a solution for making lithographic plates such as electrophotographic offset or direct-image masters hydrophilic or, in other words, an etching or dampening solution, which is mainly composed of a metal oxide, a metal sulfide and a binder resin.
The present invention relates generally to a solution for making electrophotographic offset printing plates hydrophilic and, more specifically, to a cyanogen-free desensitizing solution for offset printing, which does not contain cyanide compounds at all.
An electrophotographic offset printing plate precursor (hereinafter called the printing master) includes a photosensitive layer in which photoconductive fine powders of material such as zinc oxide is dispersed in a resin binder, and is obtained by applying ordinary electrophotographic operations to this layer to form a lipophilic image.
Generally used for offset printing is a form plate made up of a non-image area likely to be wetted by water (the hydrophilic area) and a printing area unlikely to be wetted (the lipophilic area). However, the electrographic offset printing master is made up of a hydrophobic photoconductive layer so that when it is used by itself, normal printing cannot be made, because printing ink is deposited on the non-image area as well.
Therefore, prior to printing it is required to desensitize the non-image area of the printing master to make it hydrophilic. So far, cyanogen compound-containing treating solutions containing ferrocyanides and ferricyanides as the main component and cyanogen-free treating solutions containing an ammine-cobalt complex, phytic acid (inositol hexaphosphate) and its derivative and a guanidine derivative as the main component have been proposed as such desensitizing solutions.
However, these treating solutions are still less than satisfactory. That is, the former ferrocyanide and ferricyanide-containing treating solutions have some advantages of having strong desensitizing power, being capable of forming a firm, hydrophilic film and being high in the film forming rate, but have various problems in that ferrocyanide and ferricyanide ions are so unstable to heat and light that upon exposed to light, they are colored to form precipitates which makes the desensitizing power weak, and in the process of cyanogen analysis treated with strong acids, non-toxic cyanogen complexes are detected as free cyanogen, thus offering waste water disposal and pollution problems.
In view of these considerations, on the other hand, the cyanogen-free treating solutions containing the latter desensitizing agents as the main component have been proposed in the art. However, these treating solutions are still insufficient to obtain satisfactory lithographic masters. More specifically, the latter are slower in the film forming rate than the former, and so have the disadvantage that a hydrophilic film having a physical strength high enough for immediate printing cannot be formed only by passing a plating precursor once in the processor etching manner, giving rise to scumming or degradation of dot gradation.
So far, it has been known that phytic acid and its metal derivative form metal chelate compounds, and various desensitizing agents for offset masters have been proposed in the art. However, they are all slow in the film forming rate, so that any printable, hydrophilic film cannot be formed by a single processor treatment: that is, they have the disadvantage that there is scumming or degradation of dot gradation due to unsatisfactory separability.
In order to solve the problems mentioned above, investigation has been made as to the addition of various additives to the treating solutions based on phytic acid. Specifically, there are available treating solutions to which lower amines, alkanolamines and polyamines (see, for instance, Japanese Provisional Patent Publication Nos. 54-117201, 53-109701 and 1-25994). These solutions maintain good water retention in the initial stage of use, but gets worse in terms of etching and water retention, as they are continuously used. In addition, when they are used after long-term storage, the water retention drops, making scumming likely to occur.
Furthermore, there are available treating solutions to which cation polymers are added (see, for instance, Japanese Provisional Patent Publication No. 60-23099). Like the phytic acid solutions, these solutions degrade after continued use and long-term storage and gives rise to rust as well.
In view of energy saving, on the other hand, automatic printing machines of small size with built-in desensitizing systems have be particularly spread in recent years, and the plate-making with offset masters by electrophotography have been achieved within a more reduced time than ever before. For this reason, it is now required that the desensitizing time be reduced and the life of the desensitizing solution be increased. However, these are difficult to achieve by conventional treating solutions.
A primary object of this invention is to provide a desensitizing or dampening solution for offset printing plate precursors which poses no pollution problem, can be stably used after long-term storage and continued use, and can reduce the etching time or is excellent in the desensitizing capability.